Add support for ETag in 'plain' view

When downloading a blob identified by its path, the client might want
to know if the blob has been modified since a previous download of the
same path. To this end, an ETag containing the blob SHA1 seems to be
ideal.

Todo: add support for HEAD requests...

Suggested-by: Owen Taylor <otaylor@redhat.com>
Signed-off-by: Lars Hjemli <hjemli@gmail.com>

cgit.c

@@ -206,6 +206,7 @@ static void prepare_context(struct cgit_context *ctx)
 	ctx->page.size = 0;
 	ctx->page.modified = time(NULL);
 	ctx->page.expires = ctx->page.modified;
+	ctx->page.etag = NULL;
 }
 
 struct refmatch {